1. Field of the Invention
This invention relates to testing systems, and more particularly, to the high speed testing of substrates using an electron beam.
2. Description of Related Art
Flat panel displays (FPDs) are viable alternatives to Cathode Ray Tubes (CRTs) for display of electronic information. They provide several advantages related to their small size and low power consumption. However, some manufacturing problems, such as the capability to test their expected performance during manufacturing, make them more expensive than typical CRTs. It is known that for replacing the CRT in consumer applications, such as TVs, the manufacturing costs of the FPDs must drop significantly. Currently the most popular FPD technology is the Thin Film Transistor (TFT) Liquid Crystal Display (LCD). It is used in high end laptop computers, where the price sensitivity is not as significant as in general consumer electronics.
FIG. 1 schematically illustrates a typical TFT FPD layout. A multiplicity of TFT FPDs 20 are manufactured in a single glass substrate 11 using lithographic and semiconductor processes similar to those used in the manufacture of integrated circuits. A typical FPD 20 consists of an array of pixel electrodes that are individually and repetitively activated to control the liquid crystal light emission of the panel and therefore generate a two dimensional picture. The pixels are arranged in a column and row matrix layout. During display operation, each pixel is addressed by selecting the appropriate row L.sub.R and column L.sub.C signals. Each pixel 18 contains a pixel electrode 14, a TFT 16 and a storage capacitor 22. The TFT 16 is configured as an electronic switch. The TFT gate or switch control electrode G is connected to the display row selection signal L.sub.R and the TFT source electrode S to the display column signal L.sub.C. At the time of individual pixel activation, the required voltage signal for the pixel is presented at the column line 12 and the TFT is switched on for a short time by activating the row signal 13. During that time, the storage capacitor 22 will charge to the voltage value presented on the TFT source line 15 and will maintain the voltage value until the next pixel refreshing cycle. By repeating this process to all the pixels in the display, a two dimensional image can be represented in the display.
Currently, the dominant technology for testing TFT FPD substrates is based on direct electrical measurements provided by mechanical contact probes. FIG. 2 shows one such method. The column and row activation signals of the FPD are typically taken to two edges of the panel 17. In most cases, for Electrostatic Discharge (ESD) protection reasons, the column 19 and row 23 lines are connected together with a resistive network which is later removed during the final steps of manufacture. In any case, a mechanical probe 21 is placed in contact with the particular pixel under test (PUT). The corresponding pixel row and column lines are also contacted with mechanical probes 26 and 29. These two probes generate the TFT pixel activation signals. The pixel signal is directly measured and its proper operation assessed using a multimeter type tester 32.
There are other types of mechanical contact probing techniques that do not use probe 21 but instead measure current signals generated in the row and column lines to give an indirect indication of the pixel condition. Such a system is shown in FIG. 3. In this case, the row or TFT gate signal line is contacted using probe 25 and a signal is injected using signal generator 34, while the TFT source signal line is contacted with probe 24 and a signal injected with generator 27 and currents read with multimeter type detector 33. This arrangement provides an indirect test of the condition of the PUT and does not need a mechanical contact probe directly into the PUT.
All mechanical contact probing methods have the major disadvantage that they require a large amount of mechanical contacts (one mechanical probe per row and column lines), signal generators and signal detectors. These mechanical probes are very expensive and must be completely replaced periodically. The probe replacement costs are around $100,000.
Optical test methods are also known. In this type of system all the rows and columns are activated simultaneously--by using the ESD shorting bars--and the voltage values of the pixels are recorded using a piezostrictive optical modulator that is scanned in very close proximity to the panel. This method eliminates the need for a large number of probes, but is slow and not well suited for mass production.
A well known prior art for non-contact voltage measurements is the voltage contrast phenomenon produced by an electron beam. This principle can be briefly explained with the aid of FIGS. 4A, 4B and 4C. As shown in FIG. 4A, when an electron beam 38 impinges a conductive sample 31, secondary electrons 40 are emitted from the surface. These electrons are electrostatic and are directed to a secondary electron detector 28 which converts the electron count to an electrical signal 30. If the sample 31 is connected to ground potential, the secondary electrons have an energy distribution as shown in graph V.sub.G of FIG. 4B. If the sample is electrically biased to any voltage V as FIG. 4A depicts, the energy distribution curve of the secondary electrons is proportionally shifted by the same amount V as shown in graph V.sub.X of FIG. 4B. If the secondary electron detector response is a function of the energies of the received secondary electrons, then the shift in the energy distribution will also cause a variation in the signal output 30. By measuring such a variation, the voltage V present at the sample can be inferred and the state of the sample under inspection deduced. Typically, the transfer function of a detection system of this kind is non-linear as shown in FIG. 4C, where V is the voltage at the sample and O is the signal of the detector. With a special type of detector, such as an electron spectrometer, the transfer function can be linearized to allow for a direct measurement of the voltage at the sample.
Several Publications and patents have been issued in this relatively well known prior art. Reference is made, for example, to U.S. Pat. No. 3,961,190, directed to a "Voltage Contrast Detector for a Scanning Electron Beam Instrument", by Lukianoff et al., 1976 and the paper "The Cylindrical Secondary Electron Detector as a Voltage Measuring Device in the Scanning Electron Microscope" by Ballantyne et al., Scanning Electron Microscopy/1972 (Part I).
In principle, the electron beam voltage contrast technique could be used for the inspection of the electrical signal on FPDs electrode substrates. However, speed limitations related to the small scanning area of the electron beam and the requirement of high vacuum conditions makes this technique unsuited for FPD mass production, where inspection speed is of paramount importance.
Electron beam voltage contrast, or E-Beam testing, has been successfully used in Integrated Circuit (IC) applications. In these applications a very small electron beam spot is used to create an image and to measure the voltages present in ICs in areas smaller that one micrometer. However, its use is impractical for the high speed inspection requirements during the manufacturing of FPDs. This is because of several technological limitations, one being the maximum obtainable electron beam scan area which at the very best is only a few millimeters. The near future FPDs substrates have areas approaching 1000.times.1000 millimeters. Another limitation is that the electron beam needs of a high vacuum environment impose prohibitive inspection time overhead.